Purchased the first lens last week. At first the lens was considered to be in another league than the Sigma 70-300mm 1:4.-5.6 APO through the whole range from 70 to 300mm (and the Sigma is not a bad lens). The Canon lens was tack sharp even at wide open aperture. Right until I turned the camera in portrait position and found that the corners were blurred regardless of aperture. Using the common brickwall recording on a stable tripod (Manfrotto with built-in level) with the camera mounted absolutely parallel to the wall and using a remote switch in order to avoid any vibration.
One recording in horizontal position.
One recording in vertical position.
This procedure was done at 70,135,200 and 300mm focal length at widest aperture and the whole thing done again at F13.
Tripod was then removed to another wall and the whole story repeated. More than 100 pictures were taken (sorry-I have worked for many years in an application engineering department in a German company, so I know about statistical insecurity). There was no doubt: Beginning at around 200mm the corners were blurred. Worst case was 300mm upper right corner and hardly any difference varying the aperture. My dealer changed the lens and I could start testing all over. Unfortunately there did not seem to be any difference, so I do believe the problem is a constructing item, and I could test another 10 lenses without finding one in flawless condition. I decided to keep the lens, because I mainly use the 300mm focal lenght capturing birds and they are normally in the center of the picture (hopefully) where the picture is sharp anyway.
The creeping of the lens is a little bit annoying, but else the lens is a really bargain and IS is a great help.